Medical spirometers are used for testing/measuring respiratory functions of humans, including instant flow rate during respiration (peak-flow meters) and total volume discharge or vital capacity. Fluidic elements, such as fluidic oscillators are known for their stability, linear characteristics and reliability, and are used in such spirometers.
U.S. Pat. No. 3,714,828 describes a device for measuring the pulmonary function of a patient, comprising a fluid oscillator and a digital counter. In one embodiment, a sample of the flow is diverted by a Pitot tube to the fluid oscillator. The device is designed for measuring expiratory gases from a hospital patient who has been given a volatile anesthetic.
U.S. Pat. No. 4,182,172 discloses a flow meter of fluidic oscillator type designed for measuring the ventilation of a moving human being or an animal. The flow meter is small, light and portable. The pressure drop is described as minimal but the whole flow passes through the oscillator. The flow oscillations are detected by a suitably disposed ultrasonic transmitter and receiver.
U.S. Pat. No. 4,930,357 describes a volumetric flow meter comprising a fluidic oscillator and a plurality of parallel fluid flow bypass channels. Each channel has a special flow restriction to obtain pressure drop equal to the one across the oscillator for easier calculation of the total flow. The oscillating pressure in the feedback channels of the oscillator is measured by two sensing chambers connected to the feedback channels and closed by diaphragms with transducers thereon. The other side of the diaphragms is exposed to the atmosphere.